402 



CRUSTACEA ENTOMOSTRACA. 



third and fourth pairs, nor a fifth thoracic segment distinct 

 from the stump-like abdomen ; others, for example Aclithcres, by 

 the loss of the two anterior pairs of swimming feet, sink back 

 to a still lower stage (Fig. 264). 



All the non-parasitic and many of the parasitic Copepoda 

 pass in successive moults through a larger or smaller number 

 of developmental stages, in which the still undeveloped segments 

 and appendages make their appearance, and the appendages 

 already present undergo further segmentation. Many parasitic 

 Copepoda, however, pass over the later series of nauplius forms, 



Mxf 



FIG. 264. Achtkeres percarum. a Nauplius form. 6 Larva in the youngest Cyclops stage ; 

 Kf second maxillae and Kf" maxillipeds ; c Female seen from the ventral side ; Ov, 

 ovaries ; KD cement glands ; d The smaller male seen from the side ; M :>/'. A?:r/" second 

 maxillae and maxillipeds. From Claus. 



and the larva, as soon as hatched, undergoes a moult, and appears 

 at once in the youngest Cyclops form, with antennae adapted 

 for adhering and mouth parts for piercing (Fig. 264). From 

 this stage they undergo a retrogressive metamorphosis, in which 

 they become attached to a host, lose more or less completely 

 the segmentation of the body which grows irregular in shape, 

 cast off their swimming feet, and even lose the eye, which was 

 originally present (Lernaeopoda). The males, however, in such 

 cases often remain small and dwarfed, and adhere firmly (fre- 

 quently more than one) to the body of the female in the region of 

 the genital opening (Fig. 265). 



